Various types of circuit multiplication apparatus are known in the patent literature and on the marketplace. One type of such apparatus is known as Time Assignment Speech Interpolation (TASI) Apparatus. There is described in applicant's published European Patent Application No. 25,465 and in applicant's corresponding to U.S. Pat. No. 4,523,309, TASI apparatus which has gained widespread market acceptance. The disclosure of the aforesaid European Patent Application and of U.S. Pat. No. 4,523,309 is incorporated herein by reference.
A particular feature of the abovementioned U.S. Pat. No. 4,523,309 is the fact that control information such as the assignment, synchronization and identification information is transmitted along the communication channels instead of on separate signalling channels as in the prior art. In addition, the signalling communication channel assignment information is also supplied via the voice channels rather than via the signalling channels.
There are known TASI systems in which the advantages of adaptive pulse code modulation (ADPCM) compression techniques have been incorporated, as described in a paper by Townes et al. entitled "Performance of an ADPCM/TASI System", International Conference on Communications, Seattle, Wash., June, 1980 (pgs. 42.6.1-5). Additional literature includes the paper by Agrawal, J. P. et al., entitled "The Design of an ADPCM/TASI System for PCM Speech Compression", IEEE Transactions on Communications, Vol. COM-29, No. 9 (1981).
It is pointed out in the paper by Agrawal et al. that the ADPCM technique is sensitive to the synchronism between the encoder and decoder with regard to the step sizes chosen to track the dynamic range of the prediction error. Transmission errors disturb this synchronism and a recovery time is required within which it can be reestablished.
There are no known communication systems which combine the TASI/ADPCM technique with the use of voice channels for transmission of control information. Such a combination would present a problem in maintaining the above-mentioned synchronism, since transmission of the control information represents an interruption on the voice channels.